This invention relates to a speed control system for a motor, and more particularly to improvements in motor control apparatus which is provided with a speed increase-suppressing device and which is suitable for use in elevators etc.
A control system employing semiconductor devices is extensively used for the speed control of a motor, and it is possible to execute a control which is highly reliable, precise and stable.
On the other hand, however, due to a quick response of the semiconductor, if any fault occurs in the control apparatus, there is the possibility that the motor will abnormally increase its speed in a short time. Especially in such a case where the use affects people's lives as in an elevator, the operation needs to be perfected with regard to the safety apparatus thereof.
A typical fault which causes the abnormal speed increase is the disconnection of a speed feedback circuit. The wiring itself can be prevented from being disconnected by, e.g., passing it through the interior of a pipe. However, in case where a feedback resistor in the feedback circuit is required, the disconnection of the resistor part is a problem. If the speed feedback circuit is disconnected, a speed deviation signal to enter the speed control apparatus becomes excessive, and the motor abruptly undergoes the abnormal speed increase.
As means for monitoring the abnormality of such a control system, there has been proposed a system wherein the speed deviation between a speed command and an actual speed is evaluated, and when the deviation has become greater than a predetermined value, the occurrence of the abnormality in the control system is judged, and the elevator is immediately stopped.
Even with this system, however, the fault such as the disconnection of the resistor of the speed feedback circuit is detected for the first time when the speed control apparatus provides an excessive output on account of the disconnection of the speed feedback circuit, with the result that the motor abnormally increases its speed. Therefore, the fault detection lags. Granting that there is no lag in the fault detection, a time lag is involved from the point of time of the detection to the time when a power source is turned "off" and when a brake demonstrates its braking force 100%. In case where, as in the fault of the disconnection of the speed feedback resistor, an abnormal acceleration after the fault is excessive, the elevator continues the speed increase during the time lag, and the maximum speed in the abnormal operation sometimes becomes as high as 130-150% of a rated speed.
An elevator is provided with mechanical emergency stop apparatus according to which, upon detecting by a mechanical governor that the speed of the elevator has abnormally increased to 120-130% of the rated speed, a part of an elevator car grasps a rail and is immediately stopped. The mechanical emergency stop apparatus is a protective means which is a "last resort" for the elevator. When this apparatus operates, there is the possibility that the passengers will be injured due to the sudden stop. Likewise, the elevator rail is damaged, and the restoration takes a long time. Accordingly, it is very desirable to decelerate the elevator before the mechanical emergency stop apparatus operates.
The abnormality monitoring means above described has a high probability of actuating the mechanical emergency stop apparatus, and cannot be said to be a satisfactory system in this respect.
As another expedient for monitoring the abnormality of the control system set forth above, it has been proposed to suppress the speed increase of the motor upon detecting the increase of the armature current of the motor.
This system, however, is disadvantageous in that the setting of the armature current value at which the abnormality is detected is difficult and the abnormality cannot be properly sensed.
Further, there is a third system wherein the deviation between a speed command and a speed feedback signal is entered into a blind sector circuit, and the resultant output is negatively fed back to the speed control apparatus, whereby when the speed deviation has become great, the output of the speed control apparatus is reduced, thus intending to suppress the speed increase of the motor.
In this system, there are speed deviation values which develop both in the normal operation and the abnormal operation. The abnormality cannot be properly sensed.